1. Field of the Invention
The present invention relates to a process for the production of sheet or strip steel, as well as to a sheet or strip steel particularly suitable for deep drawing.
For the deep drawing of rotationally symmetrical steel parts, the most texture-free possible cold rolled strip or sheet is preferably used, so that quasi-isotropic reforming is possible, and so that the drawn part is relatively ear-free. By relatively "ear-free", it is meant that a cylindrical deep drawn part, for example, will not have a wavy edge.
Complete absence of earing can normally only be expected when using isotropic material without segregations, without nonmetallic inclusions, without pearlite-cementite precipitations, and with a relatively pancake-free texture.
2. Background Information
In BLECH, ROHRE, PROFILE [Sheet, Tubing, Sections], No. Sep. 1977, pp. 341-346, the cause of earing is described in some detail, and a measurement of the relative ear height Z and the planar anisotropy delta r are defined. In each case, results with the value zero (or relatively ear-free material) would be ideal.
In this document, the value for the planar anisotropy is calculated from the anisotropy r for various expansion behaviors of the material in the direction of rolling, as well as at angles of 45 and 90 degrees thereto. Various adjustments of the r-values to provide different deep drawing characteristics are possible.
For the steels mentioned in the above-noted publication, ear-free material is obtained only through normalizing of the cold rolled strip in continuous annealing at approximately 1000 degrees Celsius, with the sheet, in its final condition, having a grain size of ASTM 8 with a relative ear height of approximately 0.3 to 0.4%, and with delta r being approximately .+-.0.1.
For non-normalized strip, it would be possible to achieve only a slightly eared condition, through trade-offs in processing during the production of sheet. For this, the final rolling temperature must be approximately 750 degrees Celsius, and the cold rolling reductions must be below 25% or over 80% and it is necessary to work with recrystallization temperatures of over 600 degrees Celsius, which have been shown to be unfavorable for earing.
The above-noted publication further indicates that normalizing cannot be performed in coils but only in continuous annealling, since the strips would adhere to each other at the high temperatures.
Documents which discuss the normalization of metals are U.S. Pat. No. 4,482,397, issued on Nov. 13, 1984 to Datta-Amitava and entitled "Method for Improving the Magnetic Permeability of Grain Oriented Silicon Steel"; U.S. Pat. No. 4,428,781 issued on Jan. 31, 1984 to Norstrom and entitled "Welded Steel Chain"; U.S. Pat. No. 4,054,471, issued on Oct. 18, 1977 to Datta-Amitiva and entitled "Process for Cube-On-Edge Oriented Silicon Steel"; and U.S. Pat. No. 4,030,950 issued on Jun. 21, 1977 to Schilling et al. and entitled "Process for Cube-On-Edge Oriented Boron-Bearing Silicon Steel Including Normalizing".
In German Published Patent Application No. 32 34 574, a generic cold rolled steel sheet or strip suitable for deep drawing is described. The titanium content could reportedly go as high as 0.15%, depending on the carbon, oxygen, sulfur, and nitrogen content. The winding temperature should be over 700 degrees Celsius or at least 580 degrees Celsius, with subsequent hot rolled strip warming to more than 700 degrees Celsius. In addition, a cold rolling reduction of 70 to 85%, as well as a continuous annealing at 700 to 900 degrees Celsius with a maximum of 2 minutes holding time is called for. Information on the earing of the material is not given.
From European Patent No. A1-101 740, for a generic cold rolled steel, a slab warming temperature lower than 1100 degrees Celsius, a final rolling temperature of less than Ar.sub.3, winding temperatures of 320 to 600 degrees Celsius, and a cold rolling reduction of 50 to 95%, as well as continuous recrystallization annealing, are recommended. For this, a steel with a maximum of 0.005% carbon, a maximum of 0.004% nitrogen, and a maximum of 0.02% niobium in combination with one or more of the elements aluminum, chromium, boron, or tungsten, is recommended for use. Relatively high average r-values above 1.2 are obtained. Information on the earing of the material after deep drawing is not reported.
Another process for the production of steels suitable for deep drawing, utilizing slab annealing temperatures lower than 1100 degrees Celsius, a final rolling temperature of a maximum of 780 degrees Celsius and winding temperatures of at least 450 degrees Celsius, as well as cold strip annealing in a hood type or continuous annealing furnace, is reported in European Patent No. B1-120 976. The process reportedly yields r-values near 2. Values for earing are not reported.
It is generally believed that hot rolled strip has good quasi-isotropic reformability, but has inadequate surface quality and tolerances which are too large, and, furthermore, is not produced in thicknesses less than 1.2 mm.